The study of events taking place within enamel fluid, as they are affected by external conditions, will be continued. The initial studies will relate to characterizing the lesion and its formation. The questions to be asked are: (1) How rapidly does the composition of the lesion respond to changes in the outside solution? (2) What are the Ca, PO4, and pH profiles within the lesion and in sound enamel? (3) Does the permselectivity of pellicle and plaque affect the enamel fluid composition and the caries mechanism? (4) How does F minus affect the composition and caries mechanism? (5) What are the effects of neutral salts on the solution within the lesion when pellicle and plaque are present? (6) What are the rate-controlling steps during demineralization and remineralization processes? The ultramicro techniques used in these studies are being developed and applied for the first time to the study of enamel fluid. It is expected that a significant fraction of the effort will continue to be devoted to the development and improvement of the experimental techniques.